Technical Field
The present invention relates to a valve seat insert for an internal combustion engine, and more particularly, to a valve seat insert which has excellent wear resistance and is particularly suitable for use in a high-load internal combustion engine that uses an alcohol fuel or a fuel gas.
Related art
In internal combustion engines (automotive engines) using liquid fuels such as gasoline and light oil, since lubricity between a valve and a valve seat insert is maintained to a certain extent by means of fuel or combustion products, wear of a valve seat insert is suppressed to a certain extent. However, in an engine that uses a fuel gas such as liquid petroleum gas (LPG) or compressed natural gas (CNG), or an alcohol fuel, a reduced amount of combustion products is produced, and metal-to-metal contact is likely to occur between a valve and a valve seat insert, so that wear of a valve seat insert tends to increase. Therefore, there has been a demand for a further enhancement in the wear resistance of valve seat inserts.
In regard to such a demand, for example, JP 09-242516 A describes a valve seat insert for an internal combustion engine, in which cobalt-base hard particles are dispersed in a base matrix of an iron-base alloy. It is suggested that in the valve seat insert described in JP 09-242516 A, cobalt-base hard particles is to be incorporated in an amount Of 26% to 50% into a base matrix containing, in percent (%) by weight, carbon (C) at a content of 0.5% to 1.5% and at least one element selected from the group consisting of nickel (Ni), cobalt (Co) and molybdenum (Mo) at a total content of 2.0% to 20.0%, with the balance being iron (Fe). It is disclosed that this valve seat insert is suitable for use in an internal combustion engine, such as a fuel gas engine as a representative example, in which wear caused by metal-to-metal contact is likely to occur. JP 09-242516 A describes trade name: “TRIBALLOY T-400” and trade name: “TRIBALLOY T-800” as examples of the cobalt-base hard particles to be used for the invention.
Furthermore, JP 11-12697 A describes a valve seat insert for an internal combustion engine, which contains, as base matrix components, at least carbon (C) at a content of 0.5% to 1.5% by weight and chromium (Cr) and/or vanadium (V) at a total content of 0.5% to 10.0% by weight, or even at least one element selected from the group consisting of Ni, Co and Mo at a total content of 2.0% to 20.0% by weight, with the balance being Fe, and also contains cobalt-base hard particles in an amount of 26% to 50% by weight. It is suggested that the valve seat insert for an internal combustion engine described in JP 11-12697 A can be suitably used even under harsh use conditions, as in the case of the internal combustion engine represented by a fuel gas engine. JP 11-12697 A describes hard particles having a composition containing, in percentage (%) by mass, 0.08% or less of C, 28.5% of Mo, 17.5% or Cr, and 3.4% of silicon (Si), with the balance being Co (trade name “TRIBALLOY T-800”) as preferred cobalt-base hard particles.
JP 2706561 B describes a valve seat insert material for an internal combustion engine, characterized in that the total composition has a composition containing, by weight ratio, 0.3% to 1.5% of C, 0.1% to 0.8% of Si, 1.4% to 4% of Cr, 0.1% to 2% of Ni, 2.7% to 13% of Mo, 0.2% to 9.5% of tungsten (W), 11% to 20% of Co, and 0.1% to 2.6% of V, with the balance being Fe, and that the structure has such that 20% to 50% of a high-speed tool steel phase in which metallic carbides are dispersed, 10% to 20% of a cobalt alloy hard phase in which intermetallic compounds are dispersed, an iron alloy phase containing Co—Ni—Mo—C, and an intermediate phase in which the cobalt alloy hard phase is dispersed into other phases, are mixed in a spotted manner. According to the technology described in JP 11-12697 A, high temperature wear resistance of the valve seat insert material is enhanced, and abrasive wear or fatigue fracture wear does not easily occur. Thus, enhancement of the performance of engines can be promoted. In JP 2706561 B, it is described that it is preferable to use a cobalt alloy powder having a composition containing, by weight ratio, 1.5% to 2.5 of Si, 7% to 9% of Cr, and 26% to 30% of Mo, with the balance being Co, for the use as hard particles.
Furthermore, JP 2002-285293 A describes a valve seat insert material for a high-load engine, in which the overall composition contains, by weight ratio, 12.5% to 35.3% or Co, 5.4% to 16.2% of Mo, 1.7% to 6% of Cr, 0.02% to 0.24% of V, 0.4% to 1.5% of Si, 0.01% to 13.5% of Ni, and 0.6% to 1.2% or C, with the balance being Fe and unavoidable impurities, and the valve seat insert material has a metallic structure in which a hard phase mainly containing molybdenum silicides as nuclei and being surrounded by a diffusion layer formed by Co diffused in the peripheries of the nuclei, are dispersed in bainite or in a mixed structure of bainite, sorbite, martensite and austenite. According to the technology described in JP 2002-285293 A, since the valve seat insert material exhibits excellent wear resistance, the valve seat insert material is considered promising as a valve seat insert material for a high-load engine such as a CNG engine. According to the technology described in JP 2002-285293 A, since wear resistance is imparted by dispersing a hard phase mainly containing molybdenum silicides as nuclei, it is suggested to add a Co-base alloy powder. Regarding the Co-base alloy powder, a powder containing 26% to 30% of Mo, 7% to 9% of Cr, and 2% to 3% of Si, with the balance being Co and unavoidable impurities, is mentioned as an example. It is suggested that Si in this powder is bonded to Mo and Co and forms hard molybdenum silicides and Mo—Co silicides, and thereby contributes to enhancement of wear resistance.
However, in recent: years, further performance enhancement of engines for fuel gas and the: like is pursued, and accordingly the use environment of valve seat inserts has become harsher. Thus, there is a demand for further enhancement of the wear resistance of valve seat inserts used therein. In regard to such a demand, the technologies described in JP 09-242516 A, JP 11-12697 A, JP 2706561 B, and JP 2002-285293 A have a problem that sufficiently satisfactory characteristics cannot be secured.
In connection with such a problem, for example, JP 2006-299404 A describes a valve seat insert material made of an iron-base sintered alloy for an internal combustion engine, in which hard particles containing one or two or more of an intermetallic compound containing Fe, Mo and Si as main components, an intermetallic compound containing Co, Mo and Si as main components, and an intermetallic compound containing Ni, Mo and Si as main components, and having a Vickers hardness of 500 HV0.1 to 1200 HV0.1, are dispersed in an amount of 10% to 60% by mass in a base matrix phase having a composition concerning, in percentage (%) by mass, 0.3% to 1.5% of C, and 1% to 20% in total of one or two or more selected from among Ni, Co, Mo, Cr and V, or further including one or two selected from among Cr and V; and the valve seat, insert material has a density of 6.7 g/cm3 or higher and a radial crushing strength of 350 MPa or higher. According to the technology described in JP 2006-299404 A, a large amount of hard particles having low opposite aggressibility can be stably dispersed, and even in a harsh use environment such as in a fuel gas engine, high strength and excellent wear resistance can be secured for a long time period.
Furthermore, JP 2013-113220 A describes a valve seat insert using an iron-base sintered alloy. The valve seat insert described in JP 2013-113220 A contains an iron base sintered alloy in which, before the iron-base sintered alloy to be used is mounted in a cylinder head, hard particles formed from at least one compound of intermetallic compounds, carbides, silicides, nitrides and borides of one or more elements selected from the elements of Groups 4a to 6a of the Periodic Table and having a hardness of 600 to 1600 HV, are included at an average area ratio of 5% to 45% in a cross-section, and an oxide containing tri-iron tetroxide as a main component is formed at the surface and in the interior by an oxidation treatment at an average area ratio in a cross-section of 5% to 20%. Thereby, a valve seat insert having excellent strength and wear resistance is obtained, and this is particularly suitable as a valve seat insert for a diesel engine, a LPG engine, a CNG engine or the like. Meanwhile, JP 2013-113220 A suggests that intermetallic compounds such as Fe—Mo, Fe—Cr and Co—Mo—Cr; and Fe-base alloys, Co-base alloys or Ni-base alloys, in which carbides of Cr, Mo and the like, are preferable as the hard particles formed from at least one compound selected from intermetallic compounds, carbides, silicides, nitrides and borides of one or more elements selected from the elements of Groups 4a to 6a of the Periodic Table and having a hardness of 600 to 1600 HV. However, there is no mention about the specific composition of the hard particles in JP 2013-113220 A.